Linked compressor and expander (Lincompex) systems are known in the telecommunications art. These systems have been implemented to enhance the quality of high frequency/single sideband voice links by 10 to 14 dBs. Analog voice transmission over HF and VHF/UHF links was often limited by the variability of the transmission medium, interference, and noise. The traditional way to overcome these various limitations was to increase the transmitter power. However, by increasing the transmitter power, the size, weight, power, and cost of the radio equipment also increased. On the other hand, by implementing Lincompex techniques into an HF/SSB System, the various past limitations have been overcome with a low cost lightweight radio system that offers channel quality improvement equivalent to an increase of 10 to 14 dB in apparent transmitter output power.
An example of such a Lincompex System is shown in FIGS. 1(A) and 1(B) of the present application. FIG. 1(A) shows the Lincompex modulator utilized in the transmitter of the high frequency/single sideband transmitter while FIG. 1(B) shows the digital Lincompex demodulator located in the receiver of the high frequency/single sideband receiver. These Lincompex circuits are representative of the Lincompex System taught in U.S. Pat. No. 4,271,499, entitled "Method and Apparatus for Digital Implementing a Linked Compressor-Expander Telecommunications System", to Leveque, the Inventor of the present application.
FIG. 1(A) of the present application illustrates a prior art Lincompex modulator similar to the modulator illustrated in the '499 Patent. In such a Lincompex modulator, the information to be transmitted is introduced as an input to the Lincompex modulator. The inputted information is passed through a filter 21 prior to being inputted into a compressor 23. The output of the compressor 23 is fed into a lowpass filter 25 to confine the frequency spectrum of the information below the 2900 Hz control tone. This lowpass filtered signal is then inputted into a summer 27.
The inputted signal is also received by an envelope estimate circuit 29 which outputs an amplitude signal representing the amplitude variance of a given syllable of a speech signal. The envelope estimate circuit 29 outputs this amplitude signal which is inputted to both the compressor 23 and a log circuit 31. The log circuit 31 outputs a logarithmic signal representing the logarithmic value of the amplitude signal to a FM modulator 33. The FM modulator 33 outputs a control tone to the summer 27 wherein the summer outputs a combined signal from the Lincompex modulator.
In operation, this Lincompex modulator utilizes lowpass filter 21 to confine the frequency spectrum of the information being inputted below the 2900 Hz control tone. The amplitude of the inputted signal is measured and used as a divisor for the signal waveform in the compressor 23 to form a syllabically constant waveform envelope. The logarithm of the envelope estimate modulates the frequency of a control tone with a deviation constant of 2 Hz/dB. This control tone is added to the compressed signal waveform and the sum is converted to an analog waveform for output to the transmitter.
FIG. 1(B) of the present application illustrates a prior art Lincompex demodulator similar to that illustrated in the '499 Patent. This Lincompex demodulator receives a input signal which is fed to a lowpass filter 41. The lowpass filter 41 outputs a filtered signal to a fading regulator 43. The fading regulator 43 further outputs a compensated signal to an expander 45. Moreover, this Lincompex demodulator also feeds the inputted signal into a bandpass filter 47 to recover the envelope signal. This bandpass filtered signal is further inputted into a frequency discriminator 49. The frequency discriminator 49 outputs a voltage signal to an exponential circuit 51. The exponential circuit 51 calculates an exponential value of the received voltage signal and inputs this exponential value into the expander 45.
In operation, this Lincompex demodulator receives a combined signal including a compressed signal and control tone from a radio frequency receiver which introduces the combined signal to the Lincompex demodulators input. The Lincompex demodulator, expands the compressed signal according to the extracted control tone information, and outputs an analog waveform. To achieve this result, the Lincompex demodulator isolates the compressed signal from the control tone by passing the inputted signal through a lowpass filter 41 to remove the control tone information from the combined signal. This compressed signal is further gain controlled by the fading regulator 43. The control tone is isolated from the remainder of the Lincompex demodulator input by the bandpass filter 47 and demodulated by the frequency discriminator 49 to extract its instantaneous frequency. This frequency level is transformed by an exponential circuit 51 to obtain the estimate of envelope magnitude. The compressed signal is then amplitude multiplied by the recovered envelope estimate in the expander 45 to amplitude expand the signal into the original waveform. While this processing is done in the '499 patent digitally, the digital representation of the originally encoded waveform provided at the input of the Lincompex modulator of FIG. 1(A) may be easily converted into an analog waveform for output at the same peak level as the originally encoded waveform.
By utilizing the Lincompex System, the HF link is improved as follows. First, at the transmitter where the power is limited to the available peak power, the Lincompex compressing of the peaks allows the RMS level to be increased by up to 6-9 dB. Secondly, the corresponding expansion at the demodulator has the effect of quieting noise during the periods of low speech energy and amplifying during the syllabic periods of speech. Since noise is much more noticeable during periods of speech silence, the net effect of the expansion is a perceived quality improvement of 3-6 dB.
On the other hand, it was thought that the transmission of Lincompex signals using frequency modulation would not realize any additional benefits since there is no correlation between an increase in apparent transmitter power in AM/SSB and an increase in the S/N ratio at the discriminator input in an FM system if both signals are produced by the same baseband processing techniques. In other words, the benefits received in the AM transmission would not necessarily be realized in a FM system. However, it would be be very beneficial to achieve results in a FM system similar to the results realized in the HF/SSB system because FM transmission is very desirable due to the reduction in size of the radiating elements, the use of FM transmission in multiplexing techniques, reduction of noise and interference, etc. To understand this expectation that a FM system would not benefit from Lincompex techniques, a prior art FM system will be discussed briefly.
A typical FM transmitter includes a signal input device, connected to a compressor which compresses the received input signal. The compressed signal is received by a pre-emphasis circuit which emphasizes the higher frequency components of the compressed signal by adding power to the higher frequencies which have a normally lower amplitude and thus being more susceptible to noise. This creates a greater signal to noise ratio in the higher frequencies.
An example of the operations of this pre-emphasis circuit can be seen in FIG. 2(A). In the prior art pre-emphasis circuits, as demonstrated in FIG. 2(A), the frequency components below 1 kHz are amplified at a constant level whereas the frequencies above 1 kHz are amplified to increasing degrees. The actual level of amplification for these higher frequency components is typically determined by a ratio 6 dB per octave.
After being emphasized by the pre-emphasis circuit, the emphasized compressed signal is inputted into a FM modulator. The FM modulator modulates the emphasized compressed signal with a carrier frequency to produce a frequency modulated signal. This frequency modulated signal is amplified by a radio frequency power amplifier prior to transmission over a transmission medium.
The actual characteristics of the transmitted frequency modulated signal is dependent upon the functions of the compressor. As shown in FIGS. 3(A) and 3(B), the compression ratio utilized by the compressor has a substantial effect upon the characteristics of the outputted frequency modulated signal. For example, FIG. 3(A) demonstrates the characteristics of a frequency modulated signal for a normal high peak-average voice signal with no compression. In this example, the peak deviations of the frequency modulated signal are substantially greater than the average deviation of the same signal. On the other hand, as demonstrated in FIG. 3(B), the use of a compressor can cause a frequency modulated signal to realize a greater average deviation. However, for typical compressors as demonstrated in FIG. 3(B), the average deviation is still substantially less than the peak deviation.
As is well known in the art, it is desired to have the highest possible compression ratio when utilizing frequency modulation transmission because the discriminator in the receiver outputs a voltage signal corresponding to the frequency swing of the received modulated signal, thereby the discriminator outputs a demodulated FM signal with a higher average voltage when a more compressed signal is received. This increase in the average voltage output of the discriminator is equivalent to achieving higher transmitter power in an AM signal.
As demonstrated in FIGS. 4(A) and 4(B), the discriminator output signal is increased as the compression ratio utilized in the transmitter is increased. Thus, for a given amount of noise, the signal to noise ratio will increase as the compression ratio increases. This is desired because the signal to noise ratio represents the quietness of the received signal, i.e., the higher signal to noise ratio the better the quality of the received signal and the better the quietness of the received signal.
However, while working with the Lincompex System in combination with a HF/SSB system, the Applicant discovered, contrary to expectations, that the Lincompex System can also be utilized in conjunction with a frequency modulation signal and exhibits an improvement ranging from 6 to 10 dB. When operating with the FM System, the Lincompex System allows the RMS frequency deviation to be increased without increasing the spectrum occupancy. This increases the signal component at the discriminator output. The expander follows the discriminator and quiets the noise so that both modulator and demodulator improvement factors are apparent in an FM Lincompex System. Thus, the FM Lincompex System realizes an apparent increase in the FM threshold, on a syllabic basis, as viewed by a weak signal approaching the threshold. Also, the use of the control tone concept of the Lincompex System eliminates the weak-signal "hiss" between syllabics.